Atherosclerosis is an important cause of ischemic cardiac insufficiency such as angina, myocardial infarction and the like. It has been considered that a major cause of atherosclerosis is the accumulation of cholesterol esters by foam cells which are present under the endodermis cell layer of blood vessels.
Inhibitors of acyl-CoA: cholesterol acyltransferase (hereinafter referred as ACAT) inhibit the synthesis of cholesterol esters in the foam cells and they diminish the accumulation of cholesterol esters, thereby inhibiting the formation and development of atherosclerosis caused by the accumulation of cholesterol esters.
It has also been established that atherosclerosis is linked with hypercholesterolemia. Cholesterols in food are absorbed as free cholesterol in the intestinal mucosal cell tract, esterified by ACAT and then enter the blood. Therefore, an inhibitor of ACAT will also prevent a rise in the cholesterol concentration in the blood by inhibiting the absorption of food cholesterol into the blood.
For this reason, compounds which are active in the inhibition of ACAT are useful for the treatment and prophylaxis of atherosclerosis.
Tricyclic heterocyclyl compounds which inhibit ACAT are known and, for example, a compound having the formula: ##STR2## has been disclosed in Japanese Patent Kokai Application No. Hei 2-6457.
However, it is still desired to develop a therapeutic agent having more potent activity.